Sagittal shapes of current fixed-bearing unicompartmental knee replacements differ from those of normal knees.

BACKGROUND: The principle when performing unicompartmental knee replacements (UKR), is to restore the natural alignment as well as the ligament tension. The tension in the ligaments is determined by the position of the joint line and the geometry of the articulating surfaces of the joint. If the surface geometry of the femoral component in a UKR is different from that of the natural knee it might cause abnormal ligament tension. This study was undertaken to determine the surface geometry of the native knee and to compare that with the geometry of different commercially available UKR femoral components. METHODS: Thirty-six native femurs and seven different UKR femoral component designs were included in this study. The sagittal shapes of the native femoral condyles and the prostheses were quantitatively described with the radius ratio (RR) and transition position index (TPI), which were calculated from the radii and transition point of the extension and flexion facets. RESULTS: The different prostheses showed a wider shape variability than the native medial condyles, having at least two times greater coefficient of variation for the RR and TPI. The sagittal shape of three prostheses corresponded to the native medial femoral condyles whereas five prostheses corresponded to the lateral condyles. One prosthesis had curves that fell far outside the native knee shape. CONCLUSION: There was a wider sagittal shape difference between the femoral components compared to the native knees. Clinically, the sagittal position of the prostheses can compensate for these differences, but it might be technically challenging.

Estimating regions of interest on the distal femur.

We investigated the ability of a statistical shape model to estimate unknown regions of interest related to patient-specific unicompartmental knee replacement design on the distal femur. Generality ranged between 0.67 and 1.03 mm, specificity from 0.79 to 1.07 mm, and leave-one-out root mean square estimation errors from 0.88 to 1.27 mm for different regions. Moderate to strong correlations were established between ground truths and model estimates for local morphological measurements on the medial and lateral condyles. Results compared well to similar studies in the literature, and we conclude that shape models might prove useful during patient-specific unicompartmental knee replacement design.

Application of an artificial neural network and morphing techniques in the redesign of dysplastic trochlea.

Segmentation and computer assisted design tools have the potential to test the validity of simulated surgical procedures, e.g., trochleoplasty. A repeatable measurement method for three dimensional femur models that enables quantification of knee parameters of the distal femur is presented. Fifteen healthy knees are analysed using the method to provide a training set for an artificial neural network. The aim is to use this artificial neural network for the prediction of parameter values that describe the shape of a normal trochlear groove geometry. This is achieved by feeding the artificial neural network with the unaffected parameters of a dysplastic knee. Four dysplastic knees (Type A through D) are virtually redesigned by way of morphing the groove geometries based on the suggested shape from the artificial neural network. Each of the four resulting shapes is analysed and compared to its initial dysplastic shape in terms of three anteroposterior dimensions: lateral, central and medial. For the four knees the trochlear depth is increased, the ventral trochlear prominence reduced and the sulcus angle corrected to within published normal ranges. The results show a lateral facet elevation inadequate, with a sulcus deepening or a depression trochleoplasty more beneficial to correct trochlear dysplasia.

Comparison of two commercial patellofemoral prostheses by means of computational modeling.

There are many patellofemoral prostheses designs available for patellofemoral resurfacing, but few studies provide results objectively comparing these designs. In this study two designs are compared on the basis of patella kinematics and patellofemoral kinetics by means of a computational technique. Results indicated that the patellae displaced laterally after trochlear engagement, while tilt patterns were irregular between volunteers. Patellofemoral contact loads increased with knee flexion, whereas medial patellofemoral tension diminished. The results from three volunteer-specific models showed that Prosthesis B would reproduce similar patella kinematics and patellofemoral kinetics to the baseline models. The computational technique provided a means by which prostheses designs could be compared with similar input and boundary values.

Contact stresses in a patient-specific unicompartmental knee replacement.

A custom, patient-specific unicompartmental knee replacement was developed using a unsupervised neural network trained on a database of healthy knee geometries. This custom implant was then compared to a conventional implant in terms of contact stress in a Finite Element Model. The custom implant experienced lower contact stresses at the tibiofemoral joint compared to the conventional implant. The custom implant stresses were further reduced with the use of a customized mobile bearing. The custom implant also displayed more uniform stress distribution at the bone-implant interface.

Management of overtight medial patellofemoral ligament reconstruction.

The clinical presentation of an overtight medial patellofemoral ligament (MPFL) reconstruction can differ depending on whether it is too tight in extension (extensor lag) or too tight in flexion (anterior knee pain and loss of flexion). We report one clinical case of each presentation. Both cases were treated with a percutaneous release of the graft. After the release, both patients regained a full active range of motion without residual symptoms. These complications demonstrate that the adjustment of the graft tensioning as well as its femoral position are critical steps in MPFL reconstruction. This procedure requires training and experience in order to avoid early complications related to malposition or inappropriate tensioning of the graft. A surgical management for these overtight reconstructions is recommended, as it will restore function and range of motion, and prevent late patellofemoral degeneration.

Differential forces within the proximal patellar tendon as an explanation for the characteristic lesion of patellar tendinopathy: an in vivo descriptive experimental study.

BACKGROUND: Patellar tendinopathy is a common condition affecting the posterior region of the proximal patellar tendon, but the reason for this typical location remains unclear. HYPOTHESIS: The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding anterior region. STUDY DESIGN: Descriptive laboratory study. METHOD: An optic fiber technique was used to detect forces in both the anterior and the posterior regions of the proximal patellar tendon in 7 healthy persons. The optic fiber force sensor works on the principle of the amplitude modulation of transmitted light when the optic fiber is geometrically altered owing to the forces acting on it. Longitudinal strain in the tendon or ligament produces a negative transverse strain, thus causing a force that effectively squeezes the optic fiber. Measurements were recorded during the following exercises: closed kinetic chain quadriceps contraction (eccentric and concentric), open kinetic chain quadriceps contraction (eccentric and concentric), a step exercise, and a jump exercise. RESULTS: During all the exercises, the peak differential signal output in the posterior location of the proximal patellar tendon was greater than in the corresponding anterior location. The greatest differential signal output was found in the jump and squat exercises. CONCLUSION: The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding anterior region. This finding supports the tensile-overload theory of patellar tendinopathy. CLINICAL RELEVANCE: Jump activities and deep squat exercises expose the patellar tendon to very large tendinous forces.

Recurrent patellar dislocation after medial patellofemoral ligament reconstruction.

We report on three cases of recurrent lateral patellar dislocation following a medial patellofemoral ligament (MPFL) reconstruction for patellar instability. In all three cases, an isolated MPFL reconstruction was performed with a double autogenous gracilis graft. The patellar fixation was done through bone tunnels. All three patients presented with a definite moderate to severe traumatic episode resulting in a recurrent patella dislocation and a transverse avulsion fracture at the medial rim of the patella. All three were treated by an open reduction and internal fixation with good results. No complication or recurrent dislocations occurred. We suggest that this complication is caused by the original underlying pathology such as dysplastic trochlea, abnormal TT-TG, patella alta and hyperlaxity, resulting a greater reliance upon the reconstructed MPFL for patellar stability. When subjected to a severe stress, the graft, which is stronger and stiffer than the original MPFL, will cause a fracture through the medial edge of the patella. This weak area results from the previous drill holes, which act as stress risers.

The favourable anisometry: an original concept for medial patellofemoral ligament reconstruction.

Medial patellofemoral ligament (MPFL) reconstruction for recurrent patellar dislocation has recently become more popular. We describe a technique that involves tensioning of the graft with the knee in full extension while simultaneously pulling with a bone hook on the patella in the direction of the quadriceps tendon, thus facilitating placement of the femoral attachment and accurate graft tensioning. We investigated the clinical outcome of this reconstructive procedure in 20 patients (23 knees), with a mean follow-up of 2 years. There was no recurrence of dislocation after surgery. At the 3 months follow up visit, nine knees had an extensor lag. At the last follow up visit, only one patient had an extensor lag. The mean Kujala score at the last follow-up was 93 (+/- 6) points. Severe primary chondral lesion had a negative influence on the final functional result. It is suggested that patella alta could increase the risk of post-operative extensor lag and quadriceps weakness as the tension tends to be more in the reconstructed MPFL than in the patellar tendon when the knee extends. In cases of severe patella alta we would suggest including distalisation of the tibial tubercle, with an MPFL reconstruction.

Surgical biomechanics of the patellofemoral joint.

This review presents objective data, as far as possible, about the current understanding of the biomechanics of the patellofemoral joint as it pertains to the management of patellofemoral problems. When faced with a patellofemoral malfunction, it is important to check all the soft-tissue and articular geometry factors relating to the patella locally and not to neglect the overall lower limb alignment and function. It is important to remember that small alterations in alignment can result in significant alterations in patellofemoral joint stresses and that changes in the mechanics of the patellofemoral joint can also result in changes in the tibiofemoral compartments. Surgical intervention for patellofemoral problems needs to be planned carefully and take into account an individual's anatomy.